1. Field of the Invention
This invention relates to a method of growing group III nitride semiconductor crystal layer for forming a high-quality group III nitride semiconductor crystal layer on a silicon (Si) single crystal substrate by a vapor phase growth method and to a semiconductor device incorporating a group III nitride semiconductor crystal layer formed by the growth method.
2. Description of the Prior Art
Vapor phase growth methods have heretofore been used to grow crystal layers consisting of group III nitride semiconductor represented by the general formula Al.sub.p Ga.sub.q In.sub.r N (where 0.ltoreq.p.ltoreq.1, 0.ltoreq.q.ltoreq.1, 0.ltoreq.r.ltoreq.1, p+q+r=1) on sapphire (Al.sub.2 O.sub.3 single crystal) substrate via a buffer layer also consisting of group III nitride semiconductor. In addition, light-emitting diodes (LEDs), laser diodes (LDs) and other such semiconductor devices have been fabricated using group III nitride semiconductor crystal layers formed on sapphire substrates.
Japanese Patent Public Disclosure 6-268259, for example, teaches an LED fabricated using group in nitride semiconductor crystal layers formed on a sapphire substrate. FIG. 3 shows an example configuration of an LED fabricated using a group III nitride semiconductor crystal layer formed on a sapphire substrate 201.
In FIG. 3, 202 designates a buffer layer composed of gallium nitride (GaN), 203 an n-type GaN layer, 204 an n-type clad layer composed of n-type Ga.sub.0.86 Al.sub.0.14 N, 205 a light-emitting layer composed of n-type In.sub.0.01 Ga.sub.0.99 N, 206 a p-type clad layer composed of p-type Ga.sub.0.86 Al.sub.0.14 N, 207 a contact layer composed of p-type GaN, 208 a p-type electrode, and 209 an n-type electrode.
The practice has thus been to form group III nitride semiconductor crystal layers, such as the n-type clad layer 204 composed of n-type Ga.sub.0.6 Al.sub.0.14 N and the light-emitting layer 205 composed of n-type In.sub.0.01 Ga.sub.0.99 N on the sapphire substrate 201 with, for example, the buffer layer 202 disposed therebetween. The n-type electrode 209 is formed by removing a portion of the layers 203-207 composed of group III nitride semiconductor crystal downward from the surface.
Thus, in the fabrication of LEDs using group III nitride semiconductor crystal layers formed on a sapphire substrate, the fact that the substrate is an electrical insulator has made it necessary to provide at least one of the electrodes by removing a portion of the group III nitride semiconductor crystal layers deposited on the substrate. This complicates processing and lowers product yield.
Moreover, sapphire is hard and poor in cleavage property. When semiconductor devices are fabricated using sapphire, therefore, the work of separating the substrate overlaid with the group III nitride semiconductor crystal layers into square devices is troublesome. A particular disadvantage encountered in the fabrication of LEDs using group III nitride semiconductor crystal layers formed on a sapphire substrate is that flat and smooth optical resonance facet cannot be formed at both ends of the device by utilizing cleavage surfaces.
Silicon (Si) single crystal has cubic system diamond structure.
The ability to form group III nitride semiconductor crystal layers on a substrate of silicon single crystal would simplify the work of separation into the individual square devices because it would make it possible to utilize the cleavage property of the silicon single crystal in the (011) directions for cutting the substrate overlaid with the group III nitride semiconductor crystal layers. Utilization of cleavage surfaces to form smooth optical resonance facet would also be possible in the fabrication of LDs.
Moreover, use of conductive silicon single crystal as the substrate would permit formation of an electrode on the opposite substrate surface from that on which the group III nitride semiconductor crystal layers are formed, thereby eliminating the need to effect processing for removal of a portion of the group III nitride semiconductor crystal layers at the time of electrode formation.
These considerations have led to research into technologies for forming group III nitride semiconductor crystal layers on silicon single crystal substrates by a vapor phase growth method.
A major obstacle to progress has been the lattice mismatch between silicon single crystal and group III nitride semiconductor crystal owing to the difference between their lattice constants. For example, the lattice constant of silicon single crystal is 5.431 .ANG. while the lattice constants of cubic system gallium nitride (GaN), aluminum nitride (AlN) and indium nitride (InN) are 4.51 .ANG., 4.38 .ANG. and 4.98 .ANG., respectively. Based on the lattice constant of silicon single crystal, therefore, the lattice constants of GaN, AlN and InN differ therefrom by 8-19%. This has prevented the formation of a high-quality group III nitride semiconductor crystal layer by direct growth on a silicon single crystal substrate.
Development of technologies for growing group III nitride semiconductor crystal layers on silicon single crystal substrates have therefore focused on insertion of an appropriate buffer layer between the silicon single crystal substrate and the group III nitride semiconductor crystal layer.
Electron. Lett., 33(23) (1997), pp1986-1987, for instance, describes a blue LED obtained by forming a buffer layer composed of aluminum nitride (AlN) on an n-type silicon single crystal substrate doped with antimony (Sb) and then forming a group III nitride semiconductor crystal layer on the buffer layer. The buffer layer composed of AlN and the group III nitride semiconductor crystal layer deposited thereon were formed by the molecular beam epitaxy (WRE) method.
On the other hand, Japanese Patent Public Disclosure 2-275682 teaches a technology for growing a group III nitride semiconductor crystal layer on a gallium phosphide (GaP) substrate or a silicon carbide (SiC) substrate via an interposed buffer layer of boron phosphide (BP), and says that the method is also usable in the case of utilizing a silicon single crystal substrate.
The lattice constant of BP with a zinc-blende-type crystal structure is 4.538 .ANG.. This is only slightly (about 0.6%) different from the 4.51 .ANG. lattice constant of cubic GaN. Thus, if a buffer layer consisting of flat and continuous BP can be formed on a silicon single crystal substrate, it should be easy to form a group III nitride semiconductor crystal layer with excellent crystallinity thereon.
Japanese Patent Public Disclosure 2-275682 attempts to utilize this principle by forming a buffer layer composed of BP and a group III nitride semiconductor crystal layer, both by the MOCVD method, at a temperature in the range 850-1150.degree. C. or 1200-1400.degree. C. Another generally known vapor phase growth method for crystal layers composed of BP is the halide vapor phase growth method.
However, the lattice constant of silicon crystal and BP crystal are 5.431 .ANG. and 4.538 .ANG., which amounts to a lattice constant difference of 16.5% between the two. Therefore, in actual practice, when the BP crystal layer is directly grown on the silicon single crystal substrate at a high temperature of 850.degree. C. or higher, the formed BP crystal does not become a flat and continuous layer but instead takes the form of scattered pyramid-like islands on the silicon single crystal substrate surface.
In other words, the conventional technology of using a vapor phase growth method to grow BP crystal directly on a silicon single crystal substrate at a high temperature of 850.degree. C. or higher and then growing a group III nitride semiconductor crystal layer thereon is incapable of forming a buffer layer consisting of flat and continuous BP. This makes it impossible to grow a continuous group III nitride semiconductor crystal layer of good Xi quality on the buffer layer.
Moreover, when BP crystal is grown on the surface of a silicon crystal substrate by this conventional method, the difference between the lattice constants of the two crystals makes the BP crystal highly susceptible to peeling.
One object of the present invention is therefore to provide a method of growing a group III nitride semiconductor crystal layer which, by defining the conditions for forming a buffer layer composed of flat and continuous BP on a silicon single crystal substrate by a vapor phase growth method, enables formation of a high-quality group III nitride semiconductor crystal layer on a silicon single crystal substrate.
Another object of the present invention is to provide a high-performance, easy-to-fabricate semiconductor device, e.g. an LED, utilizing a group III nitride semiconductor crystal layer formed by the growth method according to the invention.